Electro-optical device and thin film transistor and method forming the same

ABSTRACT

A method of fabricating silicon TFTs (thin-film transistors) is disclosed. The method comprises a crystallization step by laser irradiation effected after the completion of the device structure. First, amorphous silicon TFTs are fabricated. In each of the TFTs, the channel formation region, the source and drain regions are exposed to laser radiation illuminated from above or below the substrate. Then, the laser radiation is illuminated to crystallize and activate the channel formation region, and source and drain regions. After the completion of the device structure, various electrical characteristics of the TFTs are controlled. Also, the amorphous TFTs can be changed into polysilicon TFTs.

This is a Divisional application of Ser. No. 08/260,751, filed Jun. 15,1994, now U.S. Pat. No. 5,648,662, which itself is a continuation ofapplication Ser. No. 07/895,029, filed Jun. 8, 1992, now abandoned.

FIELD OF THE INVENTION

The present invention relates to an electro-optical device and a thinfilm transistor and a method for forming the same, in particular, to amethod of fabricating polycrystalline, or microcrystalline, silicon thinfilm transistors.

BACKGROUND OF THE INVENTION

One method of obtaining a polycrystalline, or microcrystalline, siliconfilm is to irradiate a completed amorphous silicon film with laserradiation, for crystallizing the amorphous silicon. This method isgenerally well known. Laser-crystallized thin-film transistorsfabricated by making use of this technique are superior to amorphoussilicon thin-film transistors in electrical characteristics includingfield effect mobility and, therefore, these laser-crystallized thin-filmtransistors are used in peripheral circuit-activating circuits foractive liquid-crystal displays, image sensors, and so forth.

The typical method of fabricating a laser-crystallized thin-filmtransistor is initiated by preparing an amorphous silicon film as astarting film. This starting film is irradiated with laser radiation tocrystallize it. Subsequently, the film undergoes a series ofmanufacturing steps to process the device structure. The most strikingfeature of the conventional manufacturing process is to carry out thecrystallization step as the initial or an intermediate step of theseries of manufacturing steps described above.

Where thin-film transistors are fabricated by this manufacturing method,the following problems take place:

(1) Since the laser crystallization operation is performed as one stepof the manufacturing process, the electrical characteristics of thethin-film transistor (TFT) cannot be evaluated until the device iscompleted. Also, it is difficult to control the characteristics.

(2) Since the laser crystallization operation is effected at thebeginning of, or during, the fabrication of the TFT, it is impossible tomodify various electrical characteristics after the device structure iscompleted. Hence, the production yield of the whole circuit system islow.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel method offabricating polycrystalline, or microcrystalline, thin film transistorswithout incurring the foregoing problems.

In accordance with the present invention, in order to make it possibleto crystallize a channel formation region and to activate the Ohmiccontact region of the source and drain by laser irradiation after thedevice structure of a thin film transistor is completed, a part of thechannel formation region and parts of the source and drain on the sideof the channel formation region are exposed to incident laser radiation.Alternatively, the source and drain regions are located on the upstreamside of the source and drain electrodes as viewed from the incidentlaser radiation, and parts of the source and drain regions are incontact with the surface of the channel formation region on which thelaser radiation impinges.

The activation of the source and drain regions is intended to impartenergy to those regions which are doped with a dopant to improve the p-or n-type characteristics, thus activating the dopant where a group IIIor V dopant atom is implanted into an intrinsic amorphous silicon filmby various methods. In this way, the electrical conductivity of the filmis improved.

Other objects and features of the invention will appear in the course ofthe description thereof which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, (a)-(d), are vertical cross sections of thin film transistorsaccording to the invention;

FIG. 2 is a plan view of a liquid-crystal display using thin filmtransistors according to the invention;

FIG. 3, (A)-(H), are cross-sectional views illustrating stepssuccessively performed to fabricate a thin film transistor according tothe invention;

FIG. 4 is a plan view of a polysilicon TFT-activated liquid-crystaldisplay having redundant configurations according to the invention; and

FIG. 5 is a circuit diagram of the redundant configurations of theliquid-crystal display shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

A TFT (thin film transistor) according to the present invention is shownin FIG. 1, (a)-(d). A doped amorphous silicon layer to become source anddrain regions and an intrinsic amorphous silicon layer to become achannel formation region can be irradiated with a laser radiation aftercompletion of the device structure as shown in FIG. 1 (a)-(d) toeffectively perform crystallization and activation thereof. In FIG.1(a), the distance between a source electrode 9 and a drain electrode 10on a TFT island is set larger than the distance between a source region11 and a drain region 12. This permits activation and crystallization ofthe source region 11, the drain region 12, and the channel formationregion 5 by laser irradiation from above the substrate.

At this time, laser radiation having a sufficient energy is irradiatedto crystallize even those portions of the intrinsic amorphous siliconlayer 5 which are under the source and drain regions, the amorphouslayer 5 becoming the channel formation region. In this manner, a channelexhibiting good characteristics can be obtained. Because the interfacebetween a gate-insulating film 4 and the channel formation region isbelow the channel formation region, the incidence of the laser radiationdoes not deteriorate the interface characteristics. Hence, thecharacteristics of the device are not impaired.

Ultraviolet radiation that is often used as a laser radiation canpenetrate through silicon oxide (SiO₂) and so if a passivation film 13consists of silicon oxide (SiO₂), then laser radiation can beilluminated from above the passivation film.

This can also prevent disturbance of the upper surface of the amorphoussilicon film due to the laser irradiation. In particular, if thepassivation film is made of silicon oxide, then this passivation filmplays the same role as a cap layer which is generally used at time ofcrystallization of an amorphous silicon film by laser irradiation. Thiscap layer is formed on the amorphous silicon film irradiated with laserradiation and prevents disturbance of the upper surface of the film whenit is irradiated with the laser radiation. Consequently, alaser-crystallized film of high quality can be obtained.

The wavelength of the laser radiation or the material of the glasssubstrate 1 is so selected that the laser radiation can penetratethrough the glass substrate 1. A base film 2 is formed on the glasssubstrate to prevent intrusion of impurities from the substrate. If thematerial of the base film 2 and the material, e.g., silicon oxide, ofthe gate insulating film pass the laser radiation, then the gateelectrode 3 is made narrower than the distance between the source anddrain electrodes to permit irradiation of the laser radiation from belowthe substrate. As a result, the doped regions under the source and drainelectrodes can be crystallized and activated. This further improvesvarious characteristics. If the base layer and the gate insulating filmare made of silicon oxide, then they act as cap layers and preventdisturbance of the lower surface of the film.

FIG. 1(b) shows an improvement over the structure shown in FIG. 1(a). Inthis structure of FIG. 1(b), the source and drain regions are moreeffectively activated by laser irradiation.

In FIG. 1(b), the source region 11 and the drain region 12 which aredoped are located on the source electrode 9 and the drain electrode 10,respectively, and are in contact with the upper surface of an intrinsicsemiconductor layer to become a channel formation region 5. Laserirradiation to the source and drain regions only from above themsuffices. Therefore, as compared with the structure shown in FIG. 1(a),various characteristics can be improved more greatly and controlled overwider ranges. In the same way as in the structure of FIG. 1(a),irradiation of laser radiation having a sufficient energy crystallizesthe portions of an intrinsic amorphous silicon layer 5 which are belowthe source and drain regions, the silicon layer 5 becoming a channelformation region. In consequence, a channel having good characteristicscan be derived. Since the interface between the gate insulating film 4and the channel formation region is below the channel formation region,the incidence of the laser radiation does not deteriorate the interfacecharacteristics. Consequently, the characteristics of the device are notdeteriorated. If the passivation film consists of silicon oxide, then itserves as a cap layer on irradiation of the laser radiation. The caplayer prevents disturbance of the upper surfaces of the doped amorphoussilicon film and of the intrinsic amorphous silicon film due to thelaser irradiation. The doped amorphous silicon film forms the source anddrain regions. The intrinsic amorphous silicon film is the channelformation region.

Structures shown in FIG. 1, (c) and (d), are totally inverted versionsof the structures of FIG. 1, (a) and (b), respectively. The wavelengthof the laser radiation or the material of the glass substrate is soselected that the laser radiation can penetrate the glass substrate. Thelaser radiation is irradiated mainly from below the substrate tocrystallize and activate the source and drain regions as well as thechannel formation region.

The structure of FIG. 1(c) is the totally inverted version of thestructure of FIG. 1(a). The distance between the source electrode 9 andthe drain electrode 10 is set larger than the distance between thesource region 11 and the drain region 12, in the same way as in thestructure of FIG. 1(a). Laser radiation is illuminated from below thesubstrate to activate and crystallize the source and drain regions andthe channel formation region 5.

At this time, laser radiation having a sufficient energy is illuminatedto crystallize even those portions of the intrinsic amorphous siliconlayer 5 which are over the source and drain regions, the amorphoussilicon layer 5 becoming a channel formation region. Thus, a channelhaving good characteristics can be obtained. Since the interface betweenthe gate insulating film and the channel formation region is locatedabove the channel formation region, the interface characteristics arenot deteriorated by the incidence of the laser radiation from below thesubstrate. In consequence, the characteristics of the device are notdeteriorated.

If the base film 2 that is formed on the glass substrate 1 to preventintrusion of impurities from the glass substrate is made of siliconoxide, then this film acts as a cap layer and prevents disturbance ofthe lower surfaces of the amorphous silicon films some of which aresource and drain regions, the remaining amorphous silicon film being achannel formation film. If the materials of the passivation film 13 andof the gate insulating film 4 pass the laser radiation such as siliconoxide, then the gate electrode 3 is made narrower than the distancebetween the source and drain electrodes. Thus, the laser radiation isirradiated from above the substrate to crystallize and activate thedoped regions located over the source and drain electrodes. Hence,various characteristics can be improved more greatly. At this time, ifthe passivation film and the gate insulating film are made of siliconoxide, they serve as cap layers and can prevent disturbance of the uppersurfaces of the amorphous silicon films due to the laser irradiation.

FIG. 1(d) shows an improvement over the structure shown in FIG. 1(c). Inthis structure of FIG. 1(d), the source and drain regions are moreeffectively activated by laser irradiation.

In FIG. 1(d), the source region 11 and the drain region 12 which aredoped are located under the source electrode 9 and the drain electrode10, respectively, and are in contact with the undersides of oppositelateral sides of a channel formation region 5. Laser irradiation to thesource and drain regions only from below them suffices. Therefore, ascompared with the structure shown in FIG. 1(c), various characteristicscan be improved more greatly and controlled over wider ranges. In thesame way as in the structure of FIG. 1(c), irradiation of laserradiation having a sufficient energy crystallizes the portions of anintrinsic amorphous silicon layer which are above the source and drainregions, the silicon layer becoming a channel formation region. Inconsequence, a channel having good characteristics can be derived.

Since the interface between the gate insulating film 4 and the channelformation region is above the channel formation region, the incidence ofthe laser radiation does not deteriorate the interface characteristics.Consequently, the characteristics of the device do not deteriorate. Ifthe base film consists of silicon oxide, then it serves as a cap layeron irradiation of the laser radiation. The cap layer preventsdisturbance of the lower surfaces of the doped amorphous silicon filmand of the intrinsic amorphous silicon film due to the laserirradiation. The doped amorphous silicon film forms the source and drainregions. The intrinsic amorphous silicon film is the channel formationregion.

In the structure described above, the source and drain regions and thechannel formation region can be activated and crystallized by laserirradiation after the device structure of the amorphous silicon thinfilm transistor has been completed.

As described thus far, laser radiation is not irradiated before orduring processing of the device structure. Rather, the device isprocessed during the fabrication of the amorphous silicon TFTs. Thelaser radiation is directed to the source, drain regions and to thechannel formation region of desired one or more of the amorphous siliconTFTs after the device structure is completed (i.e., the dopedsemiconductor layers, the intrinsic semiconductor layer to be a channellocated between source and drain regions adjacent to a gate electrodewith a gate insulating film therebetween, and the gate insulating filmare formed, the source and drain regions are formed, the source anddrain and gate electrodes are formed, and the protective film(passivation film) are formed) or after a process including formation ofconductive interconnects is completed. In this way, the channelformation region of the thin film transistor is activated, and thesource and drain regions are activated and crystallized. At this time,if the electrodes and the conductive interconnects have been completed,then the laser irradiation can be continued while monitoring electricalcharacteristics of any desired amorphous silicon TFT on the substrate ona real-time basis until an optimum value is reached. It is also possibleto fabricate thin film transistors having desired characteristics bymeasuring the electrical characteristics subsequent to laser irradiationand repeating this series of steps.

In this way, plural amorphous silicon TFTs are fabricated by the samemanufacturing method on the same substrate. After the device structureis completed, desired one or more of the amorphous silicon TFTs can bemade to have desired electrical characteristics. That is, TFTs havingdifferent electrical characteristics on the same substrate can bemanufactured by irradiation of laser radiation after the devicestructure is completed.

After plural amorphous silicon TFTs are fabricated by the samemanufacturing process on the same substrate and the device structure iscompleted, desired one or more of the TFTs are crystallized by laserirradiation. In this manner, poly-silicon TFTs are fabricated. As aresult, a system comprising the substrate on which amorphous siliconTFTs and polysilicon TFTs are fabricated can be manufactured withoutrelying on different manufacturing processes.

The laser irradiation operation is carried out in a quite short timeand, therefore, the polysilicon TFTs can be manufactured at lowtemperatures, i.e., between room temperature and 400° C., withoutsubstantially elevating the substrate temperature. This permits apolysilicon TFT system having a large area to be fabricated economicallywithout using an expensive glass such as quartz glass.

Examples of the invention are given below.

EXAMPLE 1

An integrated LCD (liquid-crystal display) system consisted of amorphoussilicon TFTs and polysilicon TFTs. As shown in FIG. 2, this integratedLCD system needed TFTs 30 for activating pixels arranged in rows andcolumns, as well as TFTs 31 for peripheral circuits. These two kinds ofTFTs were required to operate at totally different speeds. A mobility ofabout 1 cm² /V.s suffices for the TFTs for activating the pixels. On theother hand, the TFTs for the peripheral circuits must operate at a highspeed on the order of several megahertz. Preferably, therefore, the TFTsfor the activation of the pixels are made of amorphous silicon, whilethe TFTs for the peripheral circuits are made of polysilicon. In thisexample, the amorphous silicon TFTs and the polysilicon TFTs could bemanufactured simultaneously by the same process for fabricating thedevice structure of the system shown in FIG. 2.

FIG. 3, (A)-(H), schematically illustrate a method of fabricating a TFT.In this example, the structure of FIG. 1(a), or the TFT of the inversestaggered structure, was used. Of course, other structure may also beemployed. In FIG. 3(A), a sheet of glass 1 was made of an inexpensiveglass, i.e., excluding quartz glass, and withstood thermal treatmentbelow 700° C., e.g., about at 600° C. Silicon oxide (SiO₂) was depositedas a base film 2 on the glass sheet 1 up to a thickness of 1000 to 3000Å by RF (high frequency) sputtering, using a magnetron. In the presentexample, the thickness of the base film 2 was 2000 Å. The film wasformed within an ambient of 100% oxygen. The temperature was 150° C.during the formation of the film. The output of the magnetron was 400 to800W. The pressure was 0.5 Pa. A target consisting of quartz or a singlecrystal of silicon was used. The deposition rate was 30 to 100 Å/min.

A chromium (Cr) layer 3 which would become a gate electrode was formedon the base film 2 by a well-known sputtering method. The thickness ofthe film was 800 to 1000 Å. In the present example, the thickness was1000 Å. This film was patterned with a first photomask P1, resulting ina structure shown in FIG. 3(B). The gate electrode may be made oftantalum (Ta). Where the gate electrode was made of aluminum (Al), thefilm might be patterned with the first photomask P1 and then the surfacemight be anodized (anodic oxidized).

A film of silicon nitride (SiN_(x)) was formed as a gate-insulating film4 on the chromium layer 3 by PCVD. The thickness of the film was 1000 to5000 Å. In the present example, the thickness was 3000 Å. The rawmaterial gas consisted of one part of silane (SiH₄) and three parts ofammonia gas (NH₃). The film was formed at a temperature of 250° to 350°C. In the present example, the temperature was 260° C. The RF frequencywas 13.56 MHz. The RF output power was 80W. The pressure was 0.05 torr.The deposition rate was 80 Å/min.

An intrinsic amorphous silicon film 5 was formed on the gate-insulatingfilm 4 by PCVD. The thickness of the film 5 was 200 to 1000 Å. In thepresent example, the thickness was 700 Å. Silane (SiH₄) was used as theraw material gas. The film was formed at a temperature of 150° to 300°C. In the present example, the temperature was 200° C. The RF frequencywas 13.56 MHz. The RF output power was 35W. The pressure was 0.5 torr.The deposition rate was 60 Å/min.

A doped amorphous silicon layer 6 was formed on the intrinsic amorphoussilicon film 5 by PCVD. In the present example, the layer 6 was an n⁺-type amorphous silicon layer. The thickness of the layer 6 was 300 to500 Å. In the present example, the thickness was 500 Å. Silane was usedas the raw material gas. Phosphine (PH₃) which accounted for 1% of thesilane was added to implant phosphorus as an n-type dopant. The film wasformed at a temperature of 150° to 300° C. In the present example, thetemperature was 200° C. The RF frequency was 13.56 MHz. The RF outputpower was 40W. The pressure was 0.5 torr. The deposition rate was 60Å/min.

The gate insulating film 4, the intrinsic amorphous silicon layer 5, andthe n⁺ -type amorphous silicon layer 6 were formed in this way, thusresulting in a laminate shown in FIG. 3(C). These layers were allcreated by PCVD. Therefore, it is effective to form them in successionin a multi-chamber system. Other methods such as low-pressure CVD,sputtering, photo-assisted CVD may also be adopted. Subsequently, a dryetching process was performed, using a second photomask P2, to form aTFT island as shown in FIG. 3(D).

Then, a chromium layer 7 which would become source and drain electrodeswas formed by sputtering. The resulting laminate is shown in FIG. 3(E).The thickness was 500 to 1000 Å. In the present example, the thicknesswas 800 Å. This layer was patterned, using a third photomask P3. At thistime, the n⁺ -type amorphous silicon layer was patterned by dry etchingwithout peeling off resist 8. Thus, a channel formation region, a sourceelectrode 9, a drain electrode 10, a source region 11, and a drainregion 12 were formed, resulting in a laminate shown in FIG. 3(F).

A wet etching process was carried out without peeling off the resist toperform an overetching process which made the distance between thesource and drain electrodes larger than the distance between the sourceand drain regions. This made it possible to activate and crystallize thesource and drain regions by laser irradiation from above the laminate.Thereafter, the resist was peeled off, producing a laminate shown inFIG. 3(G).

As shown in FIG. 3(H), silicon oxide (SiO₂) was deposited as apassivation film on the laminate shown in FIG. 3(G) by theaforementioned RF sputtering. The thickness of this film was 1000 to3000 Å. In the present example, the thickness was 2000 Å. PCVD and othermethods may also be used. In this way, the device structure of the TFTwas completed.

Thereafter, conductive interconnects were formed. A measuring instrumentwas connected. Laser radiation was illuminated from above the devicewhile monitoring the electrical characteristics. HP-4142B was employedas the measuring instrument. An excimer laser was used as the laser. Inthis example, a KrF excimer laser producing laser radiation ofwavelength 248 nm was used. Ultraviolet radiation at this wavelengthcould penetrate through the passivation film formed at the top of theTFT. The energy E was 200 to 350 mJ/cm². The number of the shot laserpulses was 1 to 50. The temperature T_(S) of the substrate was from roomtemperature to 400° C. during the illumination.

In this way, either the channel formation region or the source and drainregions or all of them were illuminated with laser radiation whilemonitoring the electrical characteristics on the measuring instrument.As a result, the region or regions were crystallized and activated. TFTsfor activating the matrix of pixels 30 shown in FIG. 2 exhibited desiredcharacteristics. These TFTs are huge in number, e.g., 640×400=256,000.It is necessary that these TFTs have exactly the same characteristics.Hence, it has been very difficult to increase the production yield. Thenovel method could greatly reduce defective TFTs whose characteristicsdeviate greatly from intended characteristics. Consequently, theproduction yield could be enhanced greatly.

TFTs 31 (FIG. 2) for peripheral circuits were sufficiently illuminatedwith laser radiation. The result is that the characteristics wereimproved greatly. Before the laser illumination, the field effectmobility μ₁ was 0.5 to 0.8 cm² /V.s and the threshold voltage V_(th1)was 10 to 20 V. After the laser illumination, the field effect mobilityμ₂ was 10 to 100 cm² /V.s and the threshold voltage V_(th2) was 5 to 7V. In this way, the characteristics were improved greatly. The TFTscould operate at a sufficiently large velocity to activate theperipheral circuits of a liquid crystal display device. The channelformation region made of an amorphous silicon film was crystallized andbecame a polysilicon film having a high carrier mobility. The TFTs hadsatisfactory characteristics as polysilicon TFTs.

Then, ITO (indium tin oxide) was sputtered on the substrate up to athickness of 0.1 μm. Pixel electrodes were created by patterning using aphotomask. Pixel electrodes were then connected with the thin filmtransistors. This ITO film was created at a temperature between roomtemperature and 150° C. followed by an anneal within oxygen ambient orwithin the atmosphere at a temperature of 200° to 400° C. If the pixelelectrodes are not deteriorated by the laser radiation, then the TFTsmay be illuminated with laser radiation after the pixel electrodes arecompleted.

A polyimide precursor was printed on the substrate by an offset method.The laminate was sintered within a non-oxidizing ambient of, forexample, nitrogen at 350° C. for 1 hour. Then, the polyimide surface waschanged in quality by a well-known rubbing method. A means of orientingthe liquid-crystal molecules in a given direction at least at thebeginning was produced thereby. In this manner, one substrate of aliquid-crystal display was obtained.

The other substrate was created by sputtering ITO over the whole surfaceof one side of the glass substrate up to a thickness of 1 μm andpatterning the ITO into a counter electrode, using a photomask. The ITOfilm was formed at a temperature between room temperature and 150° C.,and then annealed within an oxygen or atmospheric ambient at 200° to300° C. Consequently, a second substrate was obtained.

A polyimide precursor was printed on the substrate by an offset method.The laminate was sintered at 350° C. for 1 hour within a non-oxidizingambient of, for example, nitrogen. Then, the polyimide surface waschanged in quality by a well-known rubbing method. A means of orientingthe liquid-crystal molecules in a given direction at least at thebeginning was produced thereby.

Then, a nematic-liquid crystal was sandwiched between the aforementionedtwo substrates to form an electro-optical modulating layer between thetwo substrates. Thereby, the counter electrode is provided on theelectro-optical modulating layer. External leads were bonded. Apolarizing plate was stuck to the outer surface, thus producing atransmission-type liquid-crystal display. The derived liquid-crystaldisplay was comparable to a liquid-crystal display in which TFTs foractivating a matrix of pixels are fabricated independent of peripheraldriving circuits.

As described thus far, in order to crystallize and activate the channelformation region, the source and drain regions by laser irradiationafter the completion of the device structure, it is necessary that theincident side, or the passivation film at the top of the laminate, passultraviolet radiation. In the channel formation region, it is theinterface with the gate insulating film which operates as a channel inpractice. Therefore, the intrinsic amorphous silicon layer which becomesthe channel formation region must be crystallized sufficiently. For thispurpose, this amorphous silicon layer should be made as thin aspossible.

In this way, the various characteristics can be controlled andpolysilicon TFTs can be fabricated by irradiating the amorphous siliconTFTs with laser radiation after the device structure is completed. Also,the amorphous silicon TFTs and the polysilicon TFTs can be manufacturedseparately on the same substrate during the same process for fabricatingthe same device structure. Furthermore, a circuit system having numerouspolysilicon TFTs can be fabricated at a low temperature between roomtemperature and 400° C. The system can be manufactured economicallywithout using an expensive glass such as quartz glass.

EXAMPLE 2

A polysilicon TFT liquid-crystal display having redundant circuitconfigurations was fabricated in accordance with the present invention.It is very difficult to completely operate all TFTs of a circuit systemusing numerous polysilicon TFTs such as a polysilicon TFT liquid-crystaldisplay. In reality, some form of redundant configuration is oftenadopted to improve the production yield. One kind of redundantconfiguration for amorphous silicon TFTs for activating the matrix ofpixels of an amorphous silicon TFT liquid-crystal display comprises twoTFTs connected in parallel. These two TFTs are operated simultaneously.If one of them does not operate for some cause or other, the otheroperates. This improves the production yield. However, the electriccurrent flowing through one polysilicon TFT when it is conducting is aslarge as about 0.1 mA, which is approximately 100 times as high as thecurrent of about 1 μA flowing through a conducting amorphous siliconTFT. If twice as many as TFTs are operated for redundancy, then itfollows that an exorbitant amount of current is wasted. Therefore, it isdesired that each one pixel be activated by one TFT.

Another form of redundant configuration used in a laser-crystallizedpolysilicon TFT circuit system comprises two identical polysilicon TFTcircuits which can be connected in parallel. One circuit is operated atfirst. When this circuit ceases to operate normally, or if a malfunctionis discovered at the time of an operation check, the operated circuit isswitched to the other. In this case, both circuits must be fabricatedfrom laser-crystallized polysilicon TFTs. Hence, the time of the laserirradiation operation is doubled. Also, the energy consumed is doubled.The time taken to complete the product is prolonged. In addition, thecost is increased.

These problems are solved by a novel method described now. FIG. 4 showsa polysilicon TFT liquid-crystal display having redundant configurationsaccording to the invention. As shown in FIGS. 4 and 5, in thisliquid-crystal display, all TFTs 20 for activating pixels arranged inrows and columns and a shift register 21 in a peripheral circuit haveredundant configurations. Each TFT configuration for activating a pixelhas two identical TFTs. The shift register has two identical circuits.

First, all TFTs having redundant configuration were manufactured. EachTFT assumed the structure shown in FIG. 1, (a)-(d). After the devicestructure was completed, it was irradiated with laser radiation tochange the TFTs into polysilicon TFTs. The TFTs were manufactured in thesequence shown in FIG. 3, (A)-(H), as described in connection withExample 1. A passivation film was formed. The process proceeded to thecondition in which the pixel electrodes were not yet formed, i.e., tothe step of FIG. 3(H).

FIG. 5 shows the arrangement of the TFTs for activating pixels arrangedin rows and columns. These TFTs were amorphous silicon TFTs. Two TFTswere connected in parallel. Each pair of TFTs were connected with everypixel electrode. One of these two TFTs, TFT 23 in this example, wasirradiated with laser radiation to make the TFT a polysilicon TFT. Ameasuring instrument was connected, and the operation was checked. If notrouble was found, then the operator went to the next step. Since theredundant TFT 24 was an amorphous silicon TFT, the working current wastwo orders of magnitude less than the working current through apolysilicon TFT. Therefore, the effects of redundant TFT could besubstantially neglected. Furthermore, parallel operation does not occur,because the threshold voltage for a polysilicon TFT was 0 to about 10 V,whereas the threshold voltage for an amorphous silicon TFT was about 10to 20 V. In this manner, only the laser-crystallized polysilicon TFToperated.

If any TFT does not operate normally, the redundant amorphous siliconTFT (24 in this example) forming a pair with the malfunctioning TFT isirradiated with laser radiation to make the amorphous silicon TFT apolysilicon TFT. If necessary, the interconnect to the non-redundantTFT, or point A in this example, is broken by the laser irradiation. Inthis manner, the operated TFT can be easily switched to the redundantTFT.

In this structure, if it is not necessary to switch the operated TFT tothe redundant TFT, the polysilicon TFT activating one pixel issubstantially one, though the two TFTs provide a redundantconfiguration. The electric power consumed can be halved compared withthe case in which two polysilicon TFTs are connected in parallel to forma redundant configuration. Also, the operated TFT can be switched to theredundant TFT by one or more shots of laser radiation. In consequence, avery simple and efficient method has been realized.

The redundant circuit of the shift register also comprises two samecircuits connected in parallel. After the condition shown in FIG. 3(H),the TFTs are fabricated as amorphous silicon TFTs until electrodes areconnected. Later connection or switching using a connector or the likemay also be made. All the TFTs in one circuit, 21 in this example, arecrystallized by laser irradiation. A measuring instrument is connectedto check the operation. If they operate normally, then the operator goesto the next step.

Where both circuits are connected in parallel, the TFT in the redundantcircuit is an amorphous silicon TFT and, therefore, the working currentis about two orders of magnitude smaller than the working currentthrough a polysilicon TFT. Hence, the effects of the TFT in theredundant circuit are substantially negligible. Furthermore, paralleloperation does not occur, because the threshold voltage for apolysilicon TFT is 0 to about 10 V, whereas the threshold voltage for anamorphous silicon TFT is about 10 to 20 V. In this manner, only thelaser-crystallized polysilicon TFT operates.

If the circuit does not operate normally, the amorphous silicon TFT inthe redundant circuit 22 is irradiated with laser radiation to changethe amorphous silicon TFT into a polysilicon TFT. If necessary, theinterconnect (point B in this example) to the non-redundant circuit isbroken by laser radiation. In this way, the operated circuit is easilyswitched to the redundant circuit. Of course, the connection orswitching may be made by a connector or the like.

Consequently, crystallization by laser irradiation is not needed unlessthe necessity of switching to the redundant circuit arises. The time forwhich laser radiation is irradiated is halved compared with the case inwhich all TFTs are irradiated with laser radiation at the beginning or,or during, the process for manufacturing them. As such, wastefulconsumption of energy is avoided. Also, the process time is shortened. Areduction in the cost is accomplished. Furthermore, energy is saved. Inaddition, the operation for switching the operated TFT to the redundantTFT can be quite easily performed without involving complex wiringoperations.

Moreover, this method can be applied to a device which malfunctions inuse, as well as to a device that is being manufactured. The performanceof the used device can be recovered similarly. In this manner, the novelstructure realizes an advantageous redundant configuration in a circuitcomprising a polysilicon TFT. That is, this redundant configurationoffers a reduction in the electric power consumed, a reduction in themanufacturing time, a reduction in the cost, and energy saving.

The novel manufacturing method yields the following advantages. Afterthe device structure is completed, laser radiation is illuminated whilemonitoring the electrical characteristics. This makes it easy to controlthe optimum parameter of thin film transistors forming a circuit system.In the case of a TFT liquid-crystal display or the like needing a quitelarge number of TFTs, they are required to have uniform characteristics.If the characteristics of the TFTs vary, the variations can be correctedby the novel method. Hence, the quality and the production yield can beimproved greatly. Additionally, the quality and the production yield canbe enhanced simultaneously without the need to subject the devicestructure to any complex step.

The present invention permits the various electrical characteristicssuch as the mobility of the thin film transistors on the same substrateto be controlled at will. Also, a mixed system comprising amorphoussilicon TFTs and polysilicon TFTs can be fabricated by crystallizingonly requisite TFTs.

Since the laser radiation is irradiated only for a quite short time, thesubstrate temperature is hardly elevated and so polysilicon TFT can bemanufactured at low temperatures between room temperature and 400° C. Inconsequence, a polysilicon TFT system having a large area can befabricated economically without using an expensive glass such as quartzglass.

The novel TFT structures and general TFT structures are manufactured onthe same substrate, making use of amorphous silicon. Laser radiation isdirected to the TFTs of the novel structure. Thus, a mixed systemcomprising amorphous silicon TFTs and polysilicon TFTs can be realized.

As described thus far, the novel method of manufacturing thin filmtransistors yields numerous advantages. In this way, the novel method isindustrially advantageous.

What is claimed is:
 1. A method for producing a semiconductor devicecomprising the steps of:forming a first plurality of thin filmtransistors over a substrate for constituting an active matrix circuitand a second plurality of thin film transistors over the substrate forconstituting at least first and second shift registers, each of saidfirst and second plurality of thin film transistors having an amorphoussemiconductor film as an active region thereof; irradiating a selectedpart of the second plurality of the thin film transistors with a laserlight to convert the corresponding amorphous semiconductor film into acrystalline semiconductor film while non-selected part of the secondplurality of thin film transistors are not irradiated with the laserlight, wherein one of the first and second shift registers isconstituted by the non-selected part of the thin film transistors havingthe amorphous semiconductor film and the other one of the first andsecond shift registers is constituted by the selected part of the thinfilm transistors having the crystalline semiconductor film obtained bythe irradiating step.
 2. A method for producing a semiconductor devicecomprising the steps of:forming an amorphous semiconductor filmcomprising silicon on a substrate having an insulating surface andhaving a pixel region and a peripheral circuit region; and irradiating alaser light into a portion of the semiconductor film provided in theperipheral circuit region, to crystallize the portion of thesemiconductor film; forming a plurality of thin film transistors usingthe crystallized portion of the semiconductor film and an amorphousportion of the semiconductor film, wherein the peripheral circuit regionhas the plurality of thin film transistors using the amorphous portionof the semiconductor film including the amorphous semiconductor layerand the other plurality of thin film transistors using the crystallizedportion of the semiconductor film including a semiconductor layer. 3.The method of claim 2 wherein the peripheral circuit region has at leasttwo shift registers.
 4. A method for producing a semiconductor devicecomprising the steps of:forming a pixel circuit and a peripheral circuiton a substrate having an insulating surface, the pixel circuit and theperipheral circuit each having a thin film transistor including anamorphous silicon film; and irradiating a laser light into a portion ofthe peripheral circuit, to crystallize the amorphous silicon film of thethin film transistor provided in the portion of the peripheral circuit,wherein a portion of the amorphous silicon film in the peripheralcircuit remains amorphous.
 5. A method for producing a semiconductordevice comprising the steps of:forming a plurality of thin filmtransistors each having an amorphous silicon film on a substrate havingan insulating surface having a pixel region and a peripheral circuitregion; and laser annealing a part of the thin film transistors providedin the peripheral circuit region, to obtain a crystalline silicon filmfrom the amorphous silicon film of the annealed part of the thin filmtransistors; wherein the part of the thin film transistors provided inthe peripheral circuit region comprises the crystalline silicon film andanother part of the thin film transistors provided therein comprises theamorphous silicon film.
 6. A method of manufacturing an active matrixdevice comprising the steps of:forming a plurality of first thin filmtransistors over a substrate for constituting an active matrix circuit;forming a plurality of second thin film transistors over said substrate,said second thin film transistors constituting at least first and secondperipheral circuits for driving said active matrix circuit, wherein eachof said first and second thin film transistors comprises a gateelectrode, a gate insulating film on said gate electrode, a channelregion on said gate insulating film and source and drain regions betweenwhich said channel region extends, wherein said second thin filmtransistors of one of said first and second peripheral circuits havetheir channel regions crystallized by a laser irradiation, while anotherone of said first and second peripheral circuits have their channelregions remain amorphous.
 7. The method of claim 6 wherein saidperipheral circuits include at least one shift register.
 8. A method ofmanufacturing an active matrix device comprising the steps of:forming aplurality of first thin film transistors over a substrate forconstituting an active matrix circuit; forming a plurality of secondthin film transistors over said substrate, said second thin filmtransistors constituting at least first and second peripheral circuitsfor driving said active matrix circuit, wherein each of said first andsecond thin film transistors comprises a gate electrode, a gateinsulating film on said gate electrode, a channel region on said gateinsulating film and source and drain regions between which said regionextends, wherein said second thin film transistors of one of said firstand second peripheral circuits have their channel regions crystallizedby a laser irradiation, a field effect mobility of said second thin filmtransistors after said laser irradiation is 10 to 100 cm² /Vs, whileanother one of said first and second peripheral circuits have theirchannel regions remain amorphous.
 9. The method of claim 8 wherein saidperipheral circuits include at least one shift register.
 10. A method ofmanufacturing an active matrix device comprising the steps of:forming aplurality of first thin transistors over a substrate for constituting anactive matrix circuit; forming a plurality of second thin filmtransistors over said substrate, said second thin film transistorsconstituting at least first and second peripheral circuits for drivingsaid active matrix circuit, wherein each of said first and second thinfilm transistors comprises a gate electrode, a gate insulating film, achannel region and source and drain regions between which said channelregion extends, wherein said second thin film transistors of one of saidfirst and second peripheral circuits have their channel regionscrystallized by a laser irradiation, while another one of said first andsecond peripheral circuits have their channel regions remain amorphous.11. The method of claim 10 wherein a field effect mobility of saidsecond thin film transistors after said laser irradiation is 10 to 100cm² /Vs.
 12. The method of claim 10 wherein said peripheral circuitsinclude at least one shift register.